The automatic internal defibrillator has revolutionized the management of patients with life-threatening ventricular arrhythmias. Despite the well documented benefits of implantable defibrillators, their application is far from refined. A principal limitation is the use of epicardial lead systems that restrict both ease of use as well as scope of application. Transvenous defibrillation systems offer hope for broader use but these systems are in their infancy and are not yet comparable to epicardial systems in defibrillation efficacy. If transvenous defibrillators could be inserted with the ease of a pacemaker and defibrillate as reliable as epicardial defibrillators, then procedural risk, patient stress and health care expenditures could be reduced substantially. Furthermore, the scope of application of such devices could be broadened to a more preventative role, especially in high risk patients who have yet to manifest sustained ventricular tachycardia (VT) or ventricular fibrillation (VF). The hypothesis underlying this grant is that a high-resolution finite element model of an individual thorax, derived from medical imaging data, will enable us to accurately calculate the current flow produced by stimulating electrodes placed in or on the chest. We also hypothesize that these calculations will enable us to predict the defibrillation threshold (DFT) of different electrode configurations, ultimately allowing noninvasive and rapid selection of optimal electrode placement in that individual without extensive empirical defibrillation testing. The grant will evaluate this hypothesis in experimental animals (pigs). We will optimize the performance and accuracy of the finite element model, and validate the finite element model predictions by comparing these predictions with measured values in experimental animals at extrathoracic, intrathoracic and cardiac sites. We will use the model calculations to predict the DFT of different electrode configurations, and validate these predictions in experimental animals to study the relative efficacy of different defibrillation electrode systems. Through the use of anatomically and functionally accurate 3-D finite element models that are fully validated in experimental animals, we propose endeavors to eliminate the empirical approach used in transvenous defibrillation, streamline the implantation procedure and maximize the defibrillation safety margin.